The invention relates to MOSFET sampling switches, and particularly to an improved MOSFET sampling switch that produces a constant channel resistance R.sub.DS and thereby avoids producing distortion of the signal being sampled.
FIG. 2 shows a conventional analog sampling switch that includes an N channel MOSFET 3 having its gate electrode connected to +V, its source connected by conductor 2 to receive an analog input voltage V.sub.IN, and its drain connected by sampling capacitor 5 to a conductor 7. Conductor 7 is connected to circuitry that utilizes the sampled voltage. N channel MOSFET 6 is a "grounding transistor" that grounds one plate of the sampling capacitor 5 as it is being charged to V.sub.IN through sampling MOSFET 3. The bulk electrode of MOSFET 3 is connected to -V volts. A problem with the circuit of FIG. 2 is that the gate-to-source threshold voltage (which partially determines the channel resistance R.sub.DS of MOSFET 3) is quite low, typically only a few volts. The gate-to-source voltage varies considerably as V.sub.IN varies between +5 volts and -5 volts. This gate-to-source voltage variation causes a varying channel resistance of sampling MOSFET 3. Furthermore, the threshold voltage V.sub.TN of MOSFET 3 is a strong function of the voltage between its source electrode and its body electrode. The source-to-bulk voltage varies considerably as V.sub.IN varies, producing further changes in the channel resistance of sampling MOSFET 3 as a function of the input voltage V.sub.IN. Consequently, a considerable amount of harmonic distortion is produced in the sampled version of V.sub.IN that is stored on sampling capacitor 5 by charging it through the channel resistance R.sub.DS of sampling MOSFET 3. Consequently, if conductor 7 is connected to the capacitor array of a CDAC (Capacitor Digital-to-Analog Converter), the harmonic distortion produces errors in the digital output voltage produced by the CDAC to represent the time-varying analog input voltage V.sub.IN. Conventional CMOS "transmission gates" produce harmonic distortion in an entirely similar fashion.